Cylinder-to-cylinder variations in combustion associated with air-fuel ratio imbalances may occur in engines for various reasons. For example, cylinder-to-cylinder air-fuel ratio imbalances may occur due to cylinder-to-cylinder variation in intake valve depositions, plugged exhaust gas recirculation (EGR) orifices, electrical issues, air leaks, and/or shifted fuel injectors. When an air-fuel ratio imbalance occurs in one or more cylinders, engine performance is degraded. In addition, an engine may not be able to maintain emissions compliance and fuel economy may be reduced.
One example approach for detecting air-fuel ratio imbalance is shown by Javaherian in U.S. Pat. No. 6,668,812. Therein, a time sequential series of signals are collected from an exhaust oxygen sensor over at least one engine cycle at current engine speed and load conditions, and the series of signals are converted by discrete Fourier transformation to a vector of air-fuel ratio imbalances at a specified frequency. The vector is then projected onto two fuel imbalance reference vectors of known magnitude and phase corresponding to the discrete Fourier transform of two nominal fuel imbalance patterns for the current engine speed and load. The reference vectors are previously calibrated and stored in the memory of an engine controller. An air-fuel imbalance in a cylinder is detected based on deviation of the sampled vector from the reference vector.
However, the inventors herein have recognized a potential issue with such systems. Detecting air-fuel imbalances using the method of U.S. Pat. No. 6,668,812 may be time, cost, and computation intensive due to the presence of high sampling rates and the complexity of the required vector transformation. In particular, the reliance on Fast Fourier Transformation (FFT) can result in delays in detecting and addressing air-fuel ratio imbalances.
The inventors herein have recognized the above issues and identified an approach to at least partly address the above issues. In one example, the issues described above may be addressed by a method for normalizing engine speed content at a selected frequency with respect to variations in a crankshaft angle by sampling engine speed synchronous with engine firing events, processing sampled engine speeds using a Notch filter at the selected frequency, and identifying a cylinder imbalance based on the normalized sampled signal. In this way, analysis in a frequency domain of sampled engine speed values can be done using a Notch filter, simplifying detection of cylinder imbalance, without compromising the reliability of results.
As one example, a sequential series of sampled engine speed values may be collected within an engine cycle of an engine wherein all the cylinders are aimed to be controlled at stoichiometry. The sampled values may then be filtered using a discrete Notch filter set to a frequency of once per engine cycle. Additionally, before processing the sampled values with a notch filter, the samples values may be normalized with respect to variations in crankshaft angle and stored in the memory of an engine controller. Therein, engine speed signals are sampled during the power stroke of each firing cylinder and the estimated acceleration is normalized based on the torque of each cylinder by scaling based on deviation of spark timing from MBT spark. In order to detect presence of an air/fuel imbalance in an engine cylinder, engine speed content may be sampled at multiple points within an engine firing event. The sampling frequency may be adjusted based on the engine configuration and cylinder firing frequency, and in one example is an integer multiple of engine firing frequency. The sampled data is then processed using a discrete Notch filter set to the sampling frequency and with values to cancel a once per engine cycle frequency to obtain a frequency domain characterization of engine speed. The Notch filter output is deducted from the original signal. The processed output magnitude is then compared to a threshold. Based on the magnitude relative to the threshold, the presence of a cylinder imbalance may be determined. In addition, a degree of imbalance and a directionality of the imbalance (that is, whether the imbalance is richer or leaner than stoichiometry) may also be determined based on the magnitude and phase of the processed output. Engine parameters may then be adjusted to reduce the imbalance. For example, fueling of the imbalanced cylinder may be adjusted to correct for the imbalance.
In this way, cylinder-to-cylinder variations in air/fuel ratio may be monitored. The technical effect of applying a discrete Notch filter for frequency domain characterization of engine speed content is that air/fuel ratio imbalances may be detected using faster and simpler processing methods. In particular, the need for complex, time and computation intensive processing methods, such as Fourier transformation, is reduced, without reducing the accuracy of air/fuel ratio imbalance detection. Overall, by identifying air/fuel imbalance of a cylinder with higher reliability, emissions may be reduced and engine performance may be enhanced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.